Results obtained in an analytical and kinetic study of propane or isopentane pyrolysis, either pure or in presence of trace amounts of oxygen, are reported. It is shown particularly, that oxygen accelerates or inhibits the reaction, depending upon wall conditions; an interpretation of this dual effect is proposed. The fact that the pyrolysis of these hydrocarbons can be inhibited almost completely under given conditions proves that any molecular mechanism of decomposition is negligible as compared to the chain reaction.